1. Field of the Invention
The present invention relates to a control system for a hydraulic cylinder. More particularly, the present invention relates to a control system which can stabilize a hydraulic cylinder under load.
2. Description of the Relevant Art
A hydraulic cylinder is often employed in a load lifting device, such as a crane. Fluid is supplied to, or removed from, the hydraulic cylinder to cause a piston to move within the hydraulic cylinder. Movement of the piston enables a boom of the load lifting device to lift or lower a load.
When the load is lifted, or lowered, to a desired height, an operator of the load lifting device deactivates a control to stop the fluid flow relative to the hydraulic cylinder. At this point, movement of the boom stops. Then, workers in the vicinity of the load remove, modify or otherwise interact with the load.
A natural phenomenon is known to occur once lifting or lowering of the load is stopped at the desired height. Specifically, the load will sometimes slightly lower, despite the fact that the operator has set the control to stop movement of the load. This phenomenon has been called a xe2x80x9cstick slip conditionxe2x80x9d in the art.
The xe2x80x9cstick slip conditionxe2x80x9d can be very concerning, particularly when workers are in the vicinity of the load. For example, a worker could be injured under the load, pinned between a shift in the load, or could lose their balance when the load moves.
The xe2x80x9cstick slip conditionxe2x80x9d occurs because of a cooling of the hydraulic fluid. When fluid is repeatedly pumped into, and evacuated from, hydraulic components in the system, the temperature of the fluid in the cylinder will be raised significantly. Further, the temperature of the mechanical system handling the fluid will rise. Once the operator controls the load lifting device to stop movement of the load, fluid is no longer pumped into or evacuated from the hydraulic cylinder. As the fluid and mechanical system sit idle, they cool. This causes the pressure in the hydraulic cylinder to decrease. The pressure decreases because of a change in the energy of the fluid as it cools (i.e. a thermal fluid contraction), and a change in the static friction of the mechanical system as it cools.
Eventually, the pressure in the hydraulic cylinder will decrease to a point where the force on the piston in the cylinder, due to the load attached thereto, is greater than the system""s static pressure in the hydraulic cylinder supporting the piston, plus the mechanical friction. When this occurs, the piston will move, and hence the load will slightly lower, until a new equilibrium inside the hydraulic cylinder is established. If the mechanical system static friction is large enough to support the load to a significant degree, then the piston motion that results from that static friction finally being overcome can be substantial and very sudden (i.e. the xe2x80x9cstick slip conditionxe2x80x9d). This cycle may repeat itself numerous times as the fluid continues to cool.
Once the fluid cools to the environmental temperature, the lowering cycles of the load will stop and the xe2x80x9cstick slip conditionxe2x80x9d will cease. However, in the typical operation of a crane, it would be undesirable to allow a load to remain at a desired height for the amount of time needed for the fluid to completely cool, and the possibility of a xe2x80x9cstick slip conditionxe2x80x9d to pass. Such a practice would greatly increase the time and money required in typical construction projects. Therefore, there is a need in the art for a system which can effectively reduce or eliminate the occurrence of a xe2x80x9cstick slip conditionxe2x80x9d immediately upon raising or lowering a load to a desired height.
A first solution in the background art has been to provide a pinning system. In the first solution, once the load is elevated or lowered to the desired height a physical pin is inserted through aligned holes in moveable sections of the boom to physically link the boom sections together. The weight of the load is essentially held by the pins. Hence, if the pressure in the hydraulic cylinder drops, the load will not lower.
The first solution has drawbacks. The cost and maintenance associated with a pinning system must be added to the boom. The pinning system itself adds weight to the boom. Further, the drilling of holes through the boom sections makes it necessary to enlarge the size of the boom sections in order to maintain a suitable strength for the boom sections
Another drawback of the first solution is that only a finite number of holes are provided in the boom sections. Therefore, the load has to reside at one of only a few possible heights in order for the pins to pass through the aligned holes in the moveable boom sections. Often, the closest xe2x80x9clockablexe2x80x9d height for the load is not the optimum, or even a desirable, height in a particular circumstance.
As an alternative to the pinning solution, a second solution has been proposed in the background art. In the second solution, hydraulic pressure is maintained on the piston or rod side of the hydraulic cylinder. This pressure acts to buffer or dampen any movement of the piston as the hydraulic fluid cools.
The second solution does not prevent the xe2x80x9cstick slip condition,xe2x80x9d but rather smoothes the decent of the load as its lowers, by preventing a violent downward lurch in the load. The second solution reduces the likelihood that the load will shift, and may provide some additional time for a worker in the vicinity of the load to react by getting out of the way, or maintaining their balance when working around the load.
It is an object of the present invention to address one or more of the drawbacks associated with the background art.
Further, it is an object of the present invention to provide a method and system to prevent a xe2x80x9cstick slip condition.xe2x80x9d
Further, it is an object of the present invention to provide a control system and method for operating a load lifting device which improves the safety and accuracy of its operation.
Further, it is an object of the present invention to provide a control system and method for operating a load lifting device which maintains a constant pressure in a hydraulic cylinder sufficient to hold a load at any desired height.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.